The patterning of magnetic tunnel junctions (MTJs) and the formation of vertical contacts between MTJs and metal interconnects remain challenging processes in the formation of magnetoresistive random access memory (MRAM) integrated circuits. In one method, an MTJ is formed and electrically coupled to an overlying metal interconnect by depositing a relatively thick metallic hard mask layer on top of the many layers of magnetic and nonmagnetic materials that will make up the MTJ. This hard mask layer is patterned into the shape of the desired MTJ by conventional photolithography and reactive ion etching (RIE). The MTJ layers are then patterned using the patterned hard mask layer as a self-aligned mask structure. Afterwards, an interlayer dielectric (ILD) layer is conformally deposited on the structure and polished so that it has a flat upper surface. Conventional photolithography and RIE are then used again to pattern a trench into the ILD layer in the shape of a desired interconnect feature. The trench is formed such that the thick hard mask layer protrudes into the trench. Finally, the trench is filled with a metallic material in order to form the interconnect feature. The thick hard mask layer, therefore, acts as both a self-aligned mask for patterning the MTJ as well as the means by which the MTJ is coupled to an overlying metal interconnect.
Unfortunately, the above-described approach to patterning an MTJ and electrically coupling the MTJ to a metal interconnect may result in non-ideal patterning of the MTJ element. When patterning the non-volatile MTJ materials with RIE, an MTJ may become electrically shorted to another MTJ or other nearby feature due to sidewall redeposition during the RIE process. The RIE process may also form an undesirable “foot” on the MTJ due to shadowing effects from the thick hard mask. Moreover, sidewall roughness from the use of the relatively thick hard mask layer may result in non-ideal MTJ shapes and irreproducible device behavior. As a result, the conventional manner of patterning an MTJ and coupling the MTJ to a metal interconnect may have a severe impact on the electrical and magnetic performance of the MTJ.
Accordingly, there is a need for a method of patterning an MTJ and electrically coupling the MTJ to a metal interconnect without using a relatively thick metallic hard mask layer in the manner described above.